1. Field of the Invention
The present invention relates to a thermal printing apparatus wherein a printing operation is effected by a print head which is movable along a print line relative to a recording medium while the print head is held in contact with the medium.
2. Discussion of the Prior Art
There is known a thermal printer of a type which includes a platen, a print head, printing elements and a generally thin flat electric cable. The platen bears a force which is imparted from the print head to the recording medium during a printing operation. The print head has a flat front surface facing the recording medium. The printing elements are arranged on the front surface of the head, and are electrically connected to conductors of the electric cable through a printed wiring formed on the front surface of the print head.
In the thermal printer of the type indicated above, the quality of images printed on the recording medium is enhanced as the number of the printing elements increases. However, the number of conductor strips of the printed wiring on the print head surface, and the number of conductor wires of the electric cable increase with the number of the printing elements provided on the print head. Accordingly, the width of the electric cable and the required space for laying the cable through the printer increase, and the connection of the cable becomes difficult. Further, the increase in the number of the printing elements and the consequent dimensional increase of the cable result in an increase in the required capacity of a drive source for reciprocating the print head. As a solution to these inconveniences or drawbacks, it is proposed to use a head driver which is secured on the front surface of the print head and which is connected on its input side to the electric cable and on its output side to the printed wiring connected to the printing elements. The head driver receives serial printing data, and controls the energization of the individual printing elements according to the received printing data. The use of the head driver reduces the number of the conductor wires of the cable, as compared with the number of the printing elements, and therefore permits the cable to have a comparatively small width, while the number of the printing elements is relatively large.
Since the head driver is connected to the printed wiring formed on the front surface of the print head, it is required that the head driver be mounted at the same level as the surface on which the printing elements are disposed. Consequently, the head driver projects from the front surface of the print head, by a distance equal to the thickness of the head driver. Since the thickness is relatively large, the head driver tends to easily interfere with the recording medium, feeding device for the medium, and other components adjacent to the platen. An example of a known arrangement is shown in FIG. 9, in which a paper feed roll 82 and presser roll 84 are located close to a platen 80, in order to minimize a distance A at the trailing end of a paper sheet 86 where a printing operation is impossible. In this arrangement, however, the head driver 88 interferes with the presser roll 84. Therefore, the feed roll 82 and the presser roll 84 should be located a further distance away from the platen 80, as shown in FIG. 10, in order to avoid the interference between the head driver 88 and the presser roll 84. In this case, the distance A is undesirably increased.
To minimize the distance A where the printing is impossible, another solution as indicated in FIGS. 11 and 12 is proposed. According to this solution, the feed and presser rolls 82, 84 are located relatively close to the platen 80 as shown in FIG. 9, but the head driver 88 is attached to a lower end of a vertically extending elongate front surface 94 of a print head 92, so that the rolls 82, 84 are located above the head driver 88. This arrangement suffers from increased costs of material and manufacture of the print head 92 due to an increased length of the surface 94. Further, the arrangement results in an increased load applied to the drive source for moving the print head 92 (carriage), and increased size and weight of the printer.